Electrostatic chuck bonded to base with a bond layer and method

ABSTRACT

An electrostatic chuck for holding a substrate has an electrostatic member having a dielectric covering an electrode that is chargeable to electrostatically hold the substrate. The bond layer has a metal layer that is infiltrated or brazed between the electrostatic member and the base. The base may be a composite of a ceramic and metal, the composite having a coefficient of thermal expansion within about ±30% of a coefficient of thermal expansion of the electrostatic member. The base may also have a heater.

CROSS REFERENCE

This application is a divisional of U.S. patent application Ser. No.09/307,214, filed on May 7, 1999, titled Electrostatic Chuck HavingHeater and Method by Wang, et al. which is incorporated herein byreference in its entirety.

BACKGROUND

The present invention relates to an electrostatic chuck for holding asubstrate in a chamber.

Electrostatic chucks, which use electrostatic attraction forces to holda substrate, have several advantages over mechanical and vacuum chucks.For example, electrostatic chucks reduce stress-induced cracks caused bymechanical clamps, allow processing of a larger portion of thesubstrate, and can be used in processes conducted at low pressures. Atypical electrostatic chuck comprises an electrode covered by adielectric. When the electrode is electrically charged, an opposingelectrostatic charge accumulates in the substrate and the resultantelectrostatic force holds the substrate onto the electrostatic chuck.Once the substrate is firmly held on the chuck, a plasma of gas is usedto process the substrate.

Certain newly developed plasma processes for the fabrication ofintegrated circuits are often performed at high temperatures and inhighly erosive gases. For example, processes for etching copper orplatinum are conducted at temperatures of from 250 to 600° C., comparedto temperatures of 100 to 200° C. for etching aluminum. The hightemperatures and erosive gases thermally degrade the materials used tofabricate the chucks. The high temperature requirement is met by ceramicmaterials, such as aluminum oxide (Al₂O₃) or aluminum nitride (AIN).However, it is difficult to attach the ceramic chuck to chambercomponents which are typically made from metal because the difference inthermal expansion coefficients of the ceramic and metal can result inthermal and mechanical stresses that can cause the ceramic to fractureor chip. It is desirable to have a system for fastening a ceramic chuckto a chamber without causing excessive thermal stresses between thechuck and the chamber.

In addition, the newly developed processes often require the substrateon the electrostatic chuck to be heated to temperatures higher thanthose provided by the heat load of the plasma. The high temperatures canbe obtained by using a heater, for example, the substrate can be heatedby infrared radiation from heat lamps provided outside the chamber.However, it is difficult to pass infrared radiation through the aluminumoxide or metal walls of the chamber. In another approach, as describedin U.S. Pat. No. 5,280,156, the electrostatic chuck comprises a ceramicdielectric having both the electrode and the heater embedded therein.However, operating the embedded heater at high power levels can causethe ceramic dielectric covering the electrode to microcrack from thethermal stresses generated by differences in thermal expansion betweenthe ceramic, electrode, and heater. Thus, there is a need for anelectrostatic chuck that can be heated to high temperatures withoutdamaging the chuck.

In certain processes, it is also desirable to rapidly cool the substratein order to maintain the substrate in a narrow range of temperatures,especially for etching interconnect lines that have very smalldimensions and are positioned close together. However, temperaturefluctuations occur in high power plasmas due to variations in thecoupling of RF energy and plasma ion densities across the substrate.These temperature fluctuations can cause rapid increases or decreases inthe temperature of the substrate. Also, variations in heat transferrates between the substrate and chuck can arise from the non-uniformthermal impedances of the interfaces between the substrate, chuck, andchamber. Thus, it is desirable to have an electrostatic chuck that canrapidly cool the substrate to more closely control the temperature ofthe substrate.

Another problem that frequently arises with conventional electrostaticchucks is the difficulty in forming a secure electrical connectionbetween the electrode of the electrostatic chuck and an electricalconnector that conducts a voltage to the electrode from a terminal inthe chamber. Conventional electrical connectors have spring biasedcontacts which can oxidize and form poor electrical connections to theelectrode. Moreover, electrical connections formed by soldering orbrazing the electrical connector to the electrode often result in weakjoints that can break from thermal or mechanical stresses. Thus, it isdesirable to have an electrostatic chuck with a secure and reliableelectrical connection between the electrode and electrical connector.

Yet another problem frequently arises from the vacuum seal between theelectrostatic chuck and the surface of the chamber, especially for hightemperature processes. Typically, fluid, gas, and electrical linesextend to the electrostatic chuck through vacuum sealed feedthroughs inthe chamber. In conventional chambers, the feedthroughs are vacuumsealed by polymer O-rings that are positioned in grooves extendingaround their circumference. However, the polymer O-rings often losetheir compliance and resilience at high temperatures making it difficultto maintain the integrity of the vacuum seal.

Accordingly, there is a need for an electrostatic chuck that can beoperated at high temperatures without excessive thermal or mechanicaldegradation. There is also a need for an electrostatic chuck that canheat the substrate to higher temperatures than those provided by theheat load of the plasma. There is also a need for an electrostatic chuckhaving a uniform and low thermal impedance to transfer heat to and fromthe substrate to allow rapidly heating or cooling of the substrate.There is a further need for an electrostatic chuck having a secure andreliable connection between its electrode and electrical connector.There is also a need for degradation resistant vacuum seal between theelectrostatic chuck and chamber.

SUMMARY

An electrostatic chuck for holding a substrate, the electrostatic chuckcomprising an electrostatic member comprising a dielectric covering anelectrode that is chargeable to electrostatically hold the substrate,and a base bonded to the electrostatic member by a bond layer, the basecomprising a heater capable of raising the temperature of a substrateheld on the electrostatic member by at least about 100° C.

A method of fabricating an electrostatic chuck for holding a substrate,the method comprising the steps of:

(a) forming an electrostatic member comprising a dielectric covering anelectrode that is chargeable to electrostatically hold the substrate;

(b) forming a base comprising a heater capable of raising thetemperature of a substrate held on the electrostatic member by at leastabout 100° C.; and

(c) bonding the base to the electrostatic member by a bond layer.

An electrostatic chuck for holding a substrate, the electrostatic chuckcomprising an electrostatic member comprising a dielectric covering anelectrode that is chargeable to electrostatically hold the substrate,and a base bonded to the electrostatic member by a bond layer, the basecomprising a composite of a ceramic and metal, the composite comprisinga coefficient of thermal expansion within about ±30% of a coefficient ofthermal expansion of the electrostatic member.

A method of fabricating an electrostatic chuck for holding a substrate,the method comprising the steps of:

(a) forming an electrostatic member comprising a dielectric covering anelectrode that is chargeable to electrostatically hold the substrate;

(b) forming a base comprising a composite of a ceramic and metal, thecomposite comprising a coefficient of thermal expansion within about±30% of a coefficient of thermal expansion of the electrostatic member;and

(c) bonding the base to the electrostatic member by a bond layer.

DRAWINGS

These features, aspects, and advantages of the present invention willbecome better understood with regard to the following description,appended claims, and accompanying drawings which illustrate examples ofthe invention, where:

FIG. 1 is a schematic sectional side view of a chamber showing anelectrostatic chuck according to the present invention;

FIG. 2 is a schematic sectional side view of an electrostatic chuckhaving a base comprising channels for circulating heat transfer fluid;

FIG. 3 is a graph showing the change in the coefficient of thermalexpansion of a base for increasing volume fraction of ceramic in thebase;

FIG. 4a is a schematic sectional side view of an electrostatic chuckcomprising a base comprising two components, namely a central disk andan annular ring;

FIG. 4b is a schematic top plan view of the base of FIG. 4a showing thecentral disk having carbon fibers oriented in at least two orthogonaldirections;

FIG. 5 is a schematic sectional side view of an electrostatic member, abase, and a support having channels for circulating heat transfer fluid;

FIG. 6 is a schematic sectional side view of another version of anelectrostatic chuck;

FIG. 7a is a schematic sectional side view of an electrostatic member, abase, and a support comprising a cavity that thermally isolates the basefrom a surface of a chamber;

FIG. 7b is a schematic sectional side view of another embodiment of thesupport comprising a cavity having a trapezoidal cross-section;

FIG. 7c is a schematic sectional side view of yet another embodiment ofthe support comprising a channel having a rectangular cross-section, agas inlet for supplying gas to the channel, and a gas outlet forremoving gas from the channel;

FIG. 8a is a schematic sectional side view of a portion of anelectrostatic chuck showing an electrode, electrical connector, and adisc of conducting material therebetween; and

FIG. 8b shows the electrostatic chuck of FIG. 8a after the disc ofconducting material being melted and cooled to electrically connect theelectrode to the electrical connector.

DESCRIPTION

An exemplary chamber 25 for processing a substrate 30, such as asemiconductor wafer, is illustrated in FIG. 1. The chamber 25 comprisesa ceiling 35, sidewalls 40, and a lower surface 50 on which rests anelectrostatic chuck 55 that is used to securely hold the substrate 30during processing. The chamber 25 further comprises a process gasdistributor 60 having one or more holes 65 for introducing process gasfrom a process gas supply 70 into the chamber 25. An exhaust system 75is used to exhaust spent gas and gaseous byproducts from the chamber 25and to control the pressure of gas in the chamber 25. The exhaust system75 typically comprises an exhaust conduit having a throttle valve 80,and a plurality of pumps 85 such as roughing pumps and turbomolecularpumps. The process gas is energized by coupling RF energy to the processgas in the chamber 25 (as shown) or the process gas can be energized bymicrowaves in a remote chamber adjacent to the chamber 25 (not shown).In the exemplary chamber 25, the process gas is energized to form aplasma by applying an RF current to an inductor coil 95 adjacent to theceiling 35 to inductively couple RF energy to the gas in the chamber 25.The frequency of the RF energy applied to the inductor coil 95 istypically from about 50 KHz to about 60 MHz, and more typically about13.56 MHz.

The electrostatic chuck 55 includes an electrostatic member 100comprising an electrode 105 covered by or embedded in a dielectric 115,and having a receiving surface 120 for receiving the substrate 30. Aheat transfer gas, typically helium, is supplied from a heat transfergas supply 125 and through a conduit 130 to grooves 135 in the receivingsurface 120 to enhance heat transfer rates between the substrate 30 andthe electrostatic chuck 55. The dielectric 115 comprises a material thatallows RF energy to be coupled from the electrode 105 to the plasma, andthat also serves as an insulator that allows a DC voltage applied to theelectrode 105 to electrostatically hold the substrate 30. The electrode105 of the electrostatic member 100 comprises a single electricalconductor for monopolar operation (as shown in FIG. 1) or two moreelectrically isolated conductors for bipolar operation (as shown in FIG.2). In a monopolar chuck 55, a voltage applied to the electrode 105causes electrostatic charges to accumulate in the electrode 105 or inthe dielectric 115. Energized process gas above the substrate 30provides electrically charged species having opposing polarity whichaccumulate in the substrate 30 resulting in an attractive electrostaticforces that holds the substrate 30 to the receiving surface 120 of theelectrostatic chuck 55. In a bipolar chuck 55, at least two electrodes105 a,b are maintained at different electric potentials, therebyinducing electrostatic charges in the substrate 30 that hold it to thereceiving surface 120. An electrical connector 140 electrically connectsthe electrode 105 to a voltage supply 145 to provide desired voltage tothe electrode 105 to electrostatically hold the substrate 30.Optionally, the voltage supply 145 also provides an RF voltage to theelectrode 105 to energize and accelerate the plasma species toward thesubstrate 30 by capacitively coupling RF energy to the plasma.

To operate the electrostatic chuck 55, the chamber 25 is evacuated andmaintained at a sub-atmospheric pressure. A lift pin assembly 155comprises lift pins 160 a,b that are elevated through holes 165 a,b inthe electrostatic chuck 55 by a pneumatic lift mechanism 170. A robotarm (not shown) places the substrate 30 on the lift pins 160 a,b, andthe pneumatic lift mechanism 170 lowers the substrate 30 onto thereceiving surface 120. After the substrate 30 is placed on theelectrostatic chuck 55, the electrode 105 of the electrostatic chuck iselectrically biased with respect to the substrate 30 by the voltagesupply 145 to electrostatically hold the substrate 30. The voltagesupply 145 provides a DC voltage of about 1000 to 3000 volts to theelectrode 105. Helium, is supplied through the conduits 130 to grooves135 in the receiving surface 120 at the interface between the substrate30 and the electrostatic chuck 55 to thermally couple the substrate 30to the electrostatic chuck 55. Thereafter, an energized process gas isprovided in the chamber 25 to process the substrate 30 held on thesubstrate. On completion of the process, the pneumatic lift mechanism170 raises the lift pins 160 to raise the substrate 30 off the receivingsurface 120, allowing the substrate 30 to be removed by the robotic arm(not shown). Before raising the lift pins 160, the substrate 30 iselectrically decoupled or de-chucked by dissipating the residualelectrical charges holding the substrate 30 to the electrostatic chuck55. This is accomplished, after the voltage to the electrode 105 isturned off, by grounding the electrode 105 or maintaining a plasma atanother power level to provide a path to electrical ground for theelectrostatic charges accumulated in the substrate 30.

Particular aspects of the electrostatic chuck 55 and the system forsupporting and holding the chuck 55 in the chamber 25 will now bedescribed. As shown in FIG. 2, generally, the electrostatic member 100of the electrostatic chuck 55 is supported by a base 175 that is shapedand sized to match the electrostatic member 100 to promote efficientheat transfer across the interfaces therebetween. The base 175 cancomprise channels 180 through which heat transfer fluid is circulated toraise or lower the temperature of a substrate 30 held on the receivingsurface 120 of the electrostatic member 100. This enables thetemperature of the substrate to be precisely controlled to provide moreuniform processing. A support 190 can also be provided to support thebase 175, and the support 190 rests on the surface 50 of the chamber 25.The base 175 and the support 190 secure the electrostatic chuck 55 tothe chamber 25, provide reduced levels of thermal expansion mismatch,and provide more uniform heat transfer rates across the interfacestherebetween.

Base

In one aspect of the present invention, the base 175 for supporting theelectrostatic member 100 is fabricated to have a coefficient of thermalexpansion that is sufficiently close to that of the electrostatic member100 to reduce thermal expansion stresses that would otherwise cause theelectrostatic member 100 to separate from the base 175. In this version,the base 175 comprises a composite material having a tailoredcoefficient of thermal expansion. The composite base 175 is composed ofa plurality of materials, for example, a mixture of two or morematerials, including a first material and a second material, the volumefraction of the two materials being selected so that the base 175 has acoefficient of thermal expansion that is within about ±30% of acoefficient of thermal expansion of the electrostatic member 100.Preferably, the first material is a ceramic and the second material is ametal to provide a composite material having some ductility andincreased fracture toughness.

In one version, the base 175 comprises a porous ceramic infiltrated withmolten metal. The metal fills all the pores in the ceramic when they areopen and interconnected to one another, or only some of the pores at thesurface of the porous ceramic, when the pores are not interconnectedthroughout the structure. The coefficient of thermal expansion of a base175 comprising a porous ceramic infiltrated with a molten metal istailored by varying the volume fraction of the ceramic to the metal.FIG. 3 shows the change in the coefficient of thermal expansion of thebase 175 for increasing volume fraction of ceramic based on the formulaα_(b)=(α_(m)V_(m)E_(m)+α_(c)V_(c)E_(c))/(V_(m)E_(m)+V_(c)E_(c)),

where α_(b) is the CTE for the base 175,

α_(m), V_(m), and E_(m), respectively, are the CTE, volume fraction, andYoung's modulus for the metal, and

α_(c), V_(c), and E_(c), respectively, are the CTE, volume fraction, andYoung's modulus for the ceramic material.

For example, when the electrostatic member 100 comprises dielectric 115composed of aluminum nitride, preferably, the base 175 comprises acoefficient of thermal expansion of from about 3 to about 15 ppm/° C.,and more preferably from about 4 to about 10 ppm/° C., to provide asuitable level of CTE matching between the base 175 and theelectrostatic member 100.

The ceramic material is capable of withstanding temperatures of at leastabout 400° C. and more preferably at least about 600° C. Suitableceramic materials include one or more of aluminum oxide, aluminumnitride, boron carbide, carbon, cordierite, mullite, silicon carbide,silicon nitride, silicon dioxide and zirconium oxide. Suitable metalsfor infiltrating the porous ceramic include aluminum, copper, iron,molybdenum, titanium, tungsten or alloys thereof. The porous ceramicpreferably comprises a pore volume of from about 20 to about 80 volume %to provide a sufficiently large volume for metal infiltration. In apreferred embodiment, the base 175 comprises silicon carbide (SiC)infiltrated with aluminum (Al), the volume fraction of the ceramic tothe metal being from about 20 to about 80 volume %. As the volumefraction of ceramic to metal changes, so does the thermal and mechanicalproperties of the base 175. For example, referring to Table I, it isseen that for a base 175 comprising a silicon carbide infiltrated byaluminum, the coefficient of thermal expansion and tensile strength ofthe base 175 decreases as the volume fraction of ceramic to metalincreases, while the thermal conductivity remains constant.

TABLE I VOLUME FRACTION OF CERAMIC TO METAL (%) 63% SiC 65% SiC 70% SiCCTE (ppm/° C.) 7.9-8.1 7.2-7.7 5.7-7.0 TENSILE STRENGTH (GPa) 249 205192 THERMAL CONDUCTIVITY 175 175 175 (W/mk)

In another version, the base 175 further comprises carbon fibers 200that are oriented to provide a coefficient of thermal expansion thatmatches that of the ceramic dielectric 115. For example, as shown inFIG. 4b, the base 175 can comprise a first set of carbon fibers 200 aoriented parallel to a first axis of orientation 205 a, and a second setof carbon fibers 200 b oriented parallel to a second axis of orientation205 b that is at an angle φ with respect to the first axis oforientation 205 a. Preferably, the orientation and volume fraction ofcarbon fibers 200 are selected so that the base 175 has a coefficient ofthermal expansion that is substantially isotropic in the same plane asthat of the processing surface of the substrate 30 to minimize thermalexpansion stresses on the electrostatic member 100. More preferably, thebase 175 comprises carbon fibers 200 that are oriented in a plurality oforthogonal directions. The carbon fibers 200 oriented in a particulardirection expand in the direction parallel to their axis 205 a or 205 b.Thus, orienting the carbon fibers 200 in orthogonal directions within asingle plane tends to substantially equalize their thermal expansion intwo or more different axial directions within the same plane to providea more uniform coefficient of thermal expansion within the plane. Inaddition, the base 175 can comprise carbon fibers 200 oriented in aplurality of directions within the single plane—for example, at 20, 45,or 60° intervals—to provide an even more anisotropic thermal expansionwithin the plane.

The coefficient of thermal expansion of the base 175 can be furthertailored to match that of the electrostatic member 100 by forming a base175 comprising a hybrid or plurality of component members that each havea different coefficient of thermal expansion. The overall coefficient ofthermal expansion of the base 175 depends on the expansion coefficientof the individual component members and on their linear dimensions,α_(b)=(α₁D₁+α₂(D₂−D₁))/D₂,

where α_(b) is the overall coefficient of thermal expansion of thehybrid composite base,

α₁ and α₂ are CTEs of individual component members, and

D₁ and D₂ are linear dimensions of individual component members.

Preferably, the ratio of the linear dimensions of the component membersare selected so that the coefficient of thermal expansion of the base175 is within about ±30% of the CTE of the electrostatic member 100. Thecomponents of the base 175 are shaped and sized to cooperate to achievemultifunctional properties. For example, as shown in FIGS. 4a and 4 b,the base 175 can comprise two components 210, 215 having circularsymmetry to one another to provide different coefficients of thermalexpansion at the center 220 and peripheral edge 225 of the overlyingelectrostatic chuck 55. In this version, the base 175 comprises a disk210 surrounded by an annular ring 215, each having a different averagecoefficient of thermal expansion. Both the disk 210 and the annular ring215 are made up of a porous ceramic infiltrated with metal as describedabove. However, the volume fraction of the ceramic to metal is differentin each, and one or more can comprise carbon fibers 200 in differingvolume fractions. FIG. 4b shows a base 175 having a disk 210 comprisinga composite material containing carbon fibers 200 that are oriented inat least two orthogonal directions to provide a more uniform expansioncoefficient in a plane parallel to the plane of the substrate 30. Thedisk 210 is surrounded by an annular ring 215 made of porous siliconcarbide infiltrated with metal.

In still another version, shown in FIG. 5, the base 175 comprises athermally insulating material such as a ceramic member that thermallyinsulates the electrostatic chuck 55 from the surface 50 of the chamber25 (not shown) or the support 190. In this embodiment, the support 190further comprises channels 230 for circulating heat transfer fluidtherethrough. The base 175 serves as an interposer member lying betweenthe electrostatic chuck 55 and the surface 50 of the chamber 25 orbetween the electrostatic chuck 55 and the support 190. This reduces theheat escaping from the electrostatic chuck 55 via heat conductionthrough the surface 50 of the chamber 25 to maintain the substrate 30 athigher temperatures. In addition, the base 175 enables the electrostaticchuck 55 to form a gas tight seal with an underlying support 190 orsurface 50 of the chamber 25 by use of a conventional polymer O-ring240. The O-ring 240 is typically made from a polymer, such aspolyethylene, polyurethane, polycarbonate, polystyrene, nylon,polypropylene, polyvinylchloride, fluoroethylene polymers, or silicone,all of which are susceptible to damage by high temperatures. Forexample, temperatures of over 200° C. can cause a polyimide O-ring tolose its resilience and its ability to form a seal. Because of its lowthermal conductivity, the base 175 provides a temperature differentialsufficient high to enable the electrostatic chuck 55 to be vacuum sealedto the support 190 by an O-ring 240 without degradation of the O-ring.Preferably, the base 175 comprises a thermal conductivity sufficientlylow to provide a temperature differential of at least about 100° C.between the receiving surface 120 of the electrostatic chuck 55 and thebottom surface 50 of the chamber 25 or the support 190. More preferably,the base 175 comprises a thermal conductivity of less than about 6 W/mK.

In the embodiment shown in FIG. 5, the base 175 is made from a ceramicmaterial, such as for example, aluminum oxide, aluminum nitride, boroncarbide, carbon, cordierite, mullite, silicon carbide, silicon nitride,silicon dioxide and zirconium oxide. Of these mullite and cordierite arepreferred, because they have thermal conductivities of less than about 6W/mK and coefficients of thermal expansion of about 5 ppm/° C. which isvery close to that of the dielectric 115 of the electrostatic chuck 55.Both mullite and cordierite also have a high resistance to thermalshock. Thermal shock results from the thermal stress caused by rapidheating and cooling and it can cause microcracks to occur in a materialwhich lead to structural failure. Thus, a high resistance to failurefrom thermal shock is desirable for a base 175 that is alternatelyheated and cooled by the support 190. In addition to having a highresistance to thermal shock, both mullite and cordierite have a highresistance to erosion by energized process gases making them useful inprocesses using reactive process gases, such as fluorine.

Bond Layer

In another aspect of the present invention, the base 175 is bonded orjoined to the electrostatic member 100 by a bond layer 250 made from amaterial having high thermal conductivity, as illustrated in FIG. 6. Thebond layer 250 can comprise, for example a metal, such as aluminum,copper, iron, molybdenum, titanium, tungsten or alloys thereof, toprovide more uniform heat transfer rates across the bond layer 250 whichis desirable to provide more uniform processing. The bond layer 250eliminates use of bolts for securing the electrostatic member 100 to thebase 175 and consequently reduces mechanical stresses on theelectrostatic chuck 55. Also, the bond layer 250 has a homogeneouscomposition that provides more uniform heat transfer rates across thesubstrate 30, and reduces the differences in thermal impedances thatoccur at the interface between the base 175 and the electrostatic member100. Differences in thermal impedances can occur, for example, at theinterface between the base 175 and the electrostatic member 100 that hasa rough surface with gaps and non-contact areas that have a high thermalimpedance relative to regions having smooth surfaces. The bond layer 250is especially desirable for an electrostatic chuck 55 comprising aceramic dielectric 115 which has a lower surface 252 that forms theinterface between the electrostatic member 100 and the base 175 thatoften contains microscopic gaps and fissures (not shown). Inconventional electrostatic chucks, these gaps and fissures can create athermal barrier between the electrostatic member 100 and the base 175.In contrast, in an electrostatic chuck 55 according to the presentinvention, the bond layer 250 fills the gaps and fissures to provide asmooth surface to provide more controllable and uniform heat transferrates.

Preferably, the bond layer 250 is ductile and compliant to provide aninterface that absorbs the thermal stresses arising from the thermalexpansion mismatch between the dielectric 115 of the electrostaticmember 100 and the base 175 without damaging the electrostatic chuck.While a bonded joint provides uniform heat transfer rates, it is oftendifficult for a bonded joint to withstand the thermal stresses arisingfrom differences in thermal expansion coefficients of dissimilarmaterials, such as the electrostatic member 100 and the base 175. A bondlayer 250 according to the present invention, made from a ductile andcompliant material can flex and absorb thermal stresses that arise fromthe difference in thermal expansion coefficients of the electrostaticmember 100 and the base 175. The bond layer 250 could also be made froma polymer which is compliant and able to absorb thermal stresses.However, conventional polymer materials are often eroded by erosiveplasma and process gases, and thus it is preferred to use a compliantmetal to form the bond layer 250. Also, the bond layer made of metalgenerally has a higher thermal conductivity than a bond layer made ofpolymer.

Preferably, the bond layer 250 is made by infiltrating molten metal intothe interface between the dielectric 115 and the base 175. For example,a base 175 comprising a composite of porous ceramic and metal can bebonded to the dielectric 115 of the electrostatic member 100 by a bondlayer 250 which is formed by infiltrating molten metal into the porousceramic of the dielectric 115 and base 175. During the infiltrationprocess, the molten metal reacts with the ceramic material to form aninterfacial reaction layer that forms the bond layer 250. It is believedthat the reaction layer is confined to a zone near their contactsurfaces and penetrates less than about 250 μm into each porous ceramicsurface to provide a bond layer 250 having a thickness of from about 50to about 500 μm. This method of joining the electrostatic member 100 tothe base 175 provides a strong, vacuum tight, bond layer 250 that isalso substantially free of voids and provides uniform thermal transferrates across the interface between the base 175 and the electrostaticmember 100. Furthermore, infiltration of molten metal into the porousceramic provides a relatively thin bond layer 250 that minimizes bowingof the electrostatic member 100 which would otherwise warp the receivingsurface 120 and render the electrostatic chuck 55 unusable.

In another version, the base 175 and the electrostatic member 100 arejoined together by brazing. By brazing it is meant bonding of a ceramicmember to another ceramic or metal member, using an alloy having amelting point lower than either of the members being joined. In onemethod, a thin sheet of brazing metal (not shown) is placed between theelectrostatic member 100 and the base 175. The assembled electrostaticmember 100 and base 175 is heated to allow the metal to react withsurfaces of the electrostatic member 100 and the base 175 to form thestrong ductile bond layer 250. Alternatively, the brazing metal can bedeposited directly on the surfaces to be joined and the assembledelectrostatic member 100 and base 175 heated to form the bond layer 250.The brazing metal can comprise aluminum, zinc, copper, silicon, oralloys thereof. The assembled electrostatic member 100 and base 175 areheated to a temperature sufficiently high to melt the brazing metal, butless than the temperatures that would cause softening of theelectrostatic member 100 and base 175. Generally, the electrostaticmember 100 and base 175 are heated to a temperature of up to about 600°C. for about 180 seconds to form the brazed bond layer 250.

Heater

In another aspect of the present invention, the electrostatic chuck 55comprises a heater 235 positioned below and abutting the dielectric 115of the electrostatic member 100 to heat the substrate 30. The dielectric115 diffuses the heat from the heater 235 and thereby provides moreuniform temperatures across the substrate 30. Also, the ability of theceramic material of the dielectric 115 to withstand high temperaturesallows the heater 235 to be operated at more elevated temperatures thanthat obtainable with an electrostatic chuck 55 having a polymerdielectric. A preferred heater 235 comprises a resistive heating element255 that has a resistance sufficiently high to raise the temperature ofthe substrate 30 by at least about 100° C. The resistive heating element255 can be made from tungsten, molybdenum, iron, nickel, copper, Inconelor alloys thereof. Preferably, the resistive heating element 255comprises a planar shape that is sized to match the size of thesubstrate 30 to provide a heat flux that is relatively uniform acrossthe entire backside of the substrate 30. The resistive heating element255 can be shaped as a flat coil wound in a spiral or whirl, a wiremesh, or a zig-zag shaped element. A heater power supply 260 iselectrically connected to the resistive heating element 255 to power theheater 235. The resistive heating element 255 is electrically connectedto the heater power supply 260 by heater connectors 270 a,b thatcomprise a refractory metal and are bonded to the resistive heatingelement 255 by infiltration of a metal having a relatively low meltingtemperature. The heater power supply 260 comprises a source which has apower output of from about 500 to about 3500 watts, and which can beadjusted to provide a current level that achieves a desired substratetemperature. Preferably, a temperature controller 275 is provided tomonitor the substrate temperature and adjust the output of the heater235 to maintain the substrate 30 at temperatures from about 25 to about500° C.

Preferably, the heater 235 is embedded in the base 175 rather than inthe dielectric 115 of the electrostatic member 100. Prior art chucksthat have a heater embedded in a ceramic dielectric often crack from thehigh thermal stresses generated by localized expansion of the ceramicmaterial surrounding the heater 235. In contrast, placing the heater 235below the ceramic dielectric 115 or inside the base 175 heats the base175 which uniformly heats the dielectric 115 by conduction withoutcausing excessive thermal stresses in the dielectric 115. Also, theembedded heater 235 can maintain the substrate 30 in a small range oftemperatures with more accuracy and stability than that obtained byradiative heating, because the thermal mass of the base 175 and thedielectric 115 serve as heat sinks that prevent localized temperaturefluctuations from excessively changing the temperature of the substrate30.

The substrate 30 is heated by powering the resistive heating element 255of the heater 235 by the heater power supply 260. A power level of thecurrent provided by the heater power supply 260 is adjusted by thetemperature controller 275 in relation to a measured temperature of thesubstrate 30 to raise the substrate 30 to a temperature suitable forprocessing the substrate 30. The base 175 can reduce the flow of heatfrom the electrostatic chuck 55 to the support 190 or the surface 50 ofthe chamber 25. Optionally, heat is removed from a support 190 below thebase 175 by circulating a heat transfer fluid through the channels 230in the support 190. During processing, the temperature of the substrate30 is monitored using a temperature sensor 285, such as a thermocoupleembedded in the receiving surface 120 that provides a signal to thetemperature controller 275 that controls the heater 235 to maintain thesubstrate 30 within the desired narrow temperature range. Preferably,the electrostatic chuck 55 of the present invention is able to maintainthe substrate 30 at a temperature of from about 25 to about 500° C.within a range of about ±10° C., and more preferably, within a range ofabout ±5° C.

Support

The support 190 serves to secure the electrostatic chuck 55 to thechamber 25, and also perform one or more of other functions, such asreduce thermal expansion stresses between the chuck 55, base 175, andchamber 25; serve as a thermal insulator or thermal conductor dependingupon the desired temperature of the substrate 30; and also control heattransfer rates between the substrate 30 and the chamber 25.

One version of the support 190 is adapted to reduce thermal expansionstresses between the chuck 55, base 175, and the surface 50 of thechamber 25. In this version, the support 190 is fabricated from amaterial having a coefficient of thermal expansion that is within about±30% of a coefficient of thermal expansion of the base 175. Morepreferably, the support 190 comprises a coefficient of thermal expansionof from about 2 to about 27 ppm/° C. and most preferably of from about 3to about 12 ppm/° C. The support 190 comprises a ceramic, metal, orcomposite or mixture of ceramic and metal, including by way of example,one or more of aluminum oxide, aluminum nitride, boron carbide, carbon,cordierite, mullite, silicon carbide, silicon nitride, silicon dioxide,zirconium oxide, aluminum, copper, molybdenum, titanium, tungsten,zirconium and mixtures thereof. For example, a suitable support 190 formatching the thermal expansion coefficient of a base 175 comprising acomposite of aluminum and silicon carbide (AlSiC) (which has a CTE offrom about 4 to about 10 ppm/° C.) comprises zirconium (which has a CTEof about 6 ppm/° C.).

In another version, the support 190 is bonded to the base 175 of theelectrostatic chuck 55 by a second bond layer 295 of compliant andductile material that is provided to further absorb the thermal stressesthat occur from differences in thermal expansion of the support 190 andthe base 175. The bond layer 295 also generally has a thickness of fromabout 50 to about 500 μm. The bond layer 295 is made from a metal suchas aluminum, copper, iron, molybdenum, titanium, tungsten or alloysthereof. In addition, the bond layer 295 provides an interface with amore homogeneous composition and more uniform heat transfer rates to andfrom the substrate 30. The bond layer 295 also reduces the differencesin thermal impedances that occur at the interface between the base 175and the electrostatic member 100.

Referring to FIGS. 7a to 7 c, in another version, the support 190 isadapted to thermally insulate the base 175 of the electrostatic chuck 55from the surface 50 of the chamber 25. In this version, the support 190comprises a cavity 300 that is shaped and sized to serve as a thermalbarrier that insulates the electrostatic chuck 55 from the surface 50 ofthe chamber 25. The cavity 300 is shaped and sized to provide atemperature differential that is sufficient to enable the electrostaticchuck 55 to be sealed to the surface 50 by a conventional lowtemperature vacuum seal, such as an O-ring 240. As explained above, hightemperatures can cause the polymer O-ring 240 to lose its resilience andtherefore its ability to form a seal. Preferably, the support 190 withthe cavity 300 comprises a thermal conductivity of less than about 6W/mK to control heat transfer rates from the electrostatic chuck 55.More preferably, the support 190 comprises a cavity 300 having across-sectional area that is shaped and sized to provide a temperaturedifferential of at least about 100° C. between the chuck 55 and thesurface 50 of the chamber 25 when the substrate 30 is held at atemperature of about 500° C.

Referring to FIG. 7a, the cavity 300 comprises a cross-section havingdimensions only slightly smaller than and corresponding to those of thesupport 190. Alternatively, the cavity 300 can comprise a more complexshape tailored to control the rate at which heat is removed fromdifferent portions of the base 175 to provide more uniform temperaturesacross the receiving surface 120 of the electrostatic chuck 55. Forexample, as shown in FIG. 7b, the cavity 300 can also comprise atrapezoidal cross-section to increase heat removal from the peripheraledge of the electrostatic chuck 55, when the peripheral edge issubjected to a higher heat load from the energized process gas. Inanother alternative, shown in FIG. 7c, the cavity 300 comprise anannular channel having a rectangular cross-section which allows moreheat to be removed from the center of the base 175 thereby compensatingfor a greater heat flux at the center of the electrostatic chuck 55.

Referring to FIG. 7c, the cavity 300 can further comprise a gas inlet310 a and a gas outlet 310 b for supplying and removing a gas, such ashelium, argon, nitrogen, or air to the cavity 300. By varying thepressure of the gas in the cavity 300, the amount of heat conducted fromthe substrate 30 through the support 190 can also be varied. Thepressure of the gas in the cavity 300 is regulated to maintainsubstantially uniform temperatures across the receiving surface 120 ofthe chuck 55. Typically, the pressure of the gas is less than about 50mTorr, and more preferably, the pressure of the gas is from about 2 toabout 50 mTorr.

Optionally, as illustrated in FIG. 6, the support 190 can comprisethreaded inserts 315 of a low thermal expansion alloy, such as Kovar™ orInvar™, into which bolts 320 are threaded to secure the support 190(with the electrostatic chuck 55 bonded thereto) to the chamber 15. Thethreaded inserts 315 provide greater resilience and compliance than thebrittle material of a ceramic support 190 and are more easily machinedto provide threads for receiving the bolts 320. Alternatively, thesupport 190 is secured in the chamber 25 by a clamping ring 325, asshown in FIG. 1. The clamping ring 325 allows movement due todifferences in thermal expansion of the support 190 and the surface 50of the chamber 25, thereby preventing warping or cracking of the support190 and improving the reliability of the vacuum seal between the support190 and the surface 50. Also, any mechanical stresses induced byconventional mounting bolts made of metal are reduced, thereby extendingthe operating life of the electrostatic chuck 55 and support 190. In yetanother embodiment, shown in FIGS. 7a to 7 c, one or more of theclamping ring 325, the base 175, or the support 190 comprise a curvedsurface 330 which further reduces the mechanical stresses on theelectrostatic chuck 55 and the support 190 by distributing a clampingforce over a larger area.

Method of Fabrication

In another aspect, the present invention is directed to a method offabricating an electrostatic chuck 55 comprising an electrostatic member100 having an electrode 105 covered by a dielectric 115, a base 175joined to the electrostatic member 100, and, optionally, a heater 235. Apreferred method of fabricating the electrostatic chuck 55 will now bedescribed; however, other methods of fabrication can be used to form theelectrostatic chuck 55 and the present invention should not be limitedto the illustrative methods described herein.

Forming the Electrostatic Member

The dielectric 115 of the electrostatic member 100 comprises a ceramicor polymer material. Suitable high temperature materials includeceramics such as for example, one or more of aluminum oxide, aluminumnitride, silicon nitride, silicon dioxide, titanium dioxide, zirconiumoxide, or mixtures thereof. Generally, aluminum nitride is preferred forits high thermal conductivity which provides high heat transfer ratesfrom the substrate 30 to the electrostatic chuck 55. Also, aluminumnitride has a low CTE of about 5.5 ppm/° C. which closely matches a CTEof an electrode 105 made of molybdenum which has a CTE of about 5.1ppm/° C. Also, aluminum nitride exhibits good chemical resistance inerosive environments, especially halogen containing plasma environments.The dielectric 115 is formed by freeze casting, injection molding,pressure-forming, thermal spraying, or sintering a ceramic block withthe electrode 105 embedded therein. Preferably, a ceramic powder isformed into a coherent mass in a pressure forming process by applicationof a high pressure and a temperature. Suitable pressure formingapparatuses include an autoclave, a platen press, or an isostatic press,as for example, described in U.S. patent application Ser. No. 08/965,690filed Nov. 6, 1997; which is incorporated herein by reference.

The electrode 105 of the electrostatic member 100 comprises a refractorymetal capable of withstanding high temperatures, such as temperatures ofat least about 1500° C. Suitable metals include, for example, tungsten,molybdenum, titanium, nickel, tantalum, molybdenum or alloys thereof.Preferably, the electrode 105 is made of molybdenum, which has a thermalconductivity of about 138 W/mK, which is substantially higher than thatof most metals and alloys commonly used for electrodes 105 and enhancesheat transfer rates through the electrostatic member 100. In theembodiment shown in FIG. 6, the electrode 105 comprises a thin meshwhich is embedded in the dielectric 115 and is shaped and sizeddepending upon the shape and size of the substrate 30.

In a preferred method of forming an electrostatic member 100 with anembedded electrode 105, an isostatic press is used to apply a uniformpressure over the entire surface of the electrostatic member (notshown). A typical isostatic press comprises a pressure resistant steelchamber having a pressurized fluid for applying a pressure on anisostatic molding bag. A powdered precursor comprising a suitableceramic compound mixed with an organic binder, such as polyvinylalcohol, is packed around the electrode 105 in the isostatic molding bagand the bag is inserted in the isostatic press. The fluid in thepressure chamber is pressurized to apply a pressure on the ceramicmaterial. It is desirable to simultaneously remove air trapped in theisostatic molding bag using a vacuum pump to increase the cohesion ofthe powdered precursor. The unitary ceramic preform comprising adielectric 115 having an electrode 105 therein is removed from themolding bag and sintered to form an electrostatic member 100 with anembedded electrode 105. The gas flow conduits 130 are subsequentlyformed in the electrostatic member 100 by drilling, boring, or milling;or they can be formed by placing suitable inserts in the ceramic preformduring the molding process. After the electrostatic member 100 isformed, the receiving surface 120 is ground to obtain a flat surface toefficiently thermally couple the substrate 30 to the electrostaticchuck.

The electrical connector 140 is electrically connected to the electrode105 of the electrostatic chuck 55 to conduct an electrical charge to theelectrode 105 from a voltage supply terminal 340 in the chamber 25. Theelectrical connector 140 is also made of a refractory metal having amelting temperature of at least about 1500° C. Suitable metals include,for example, tungsten, titanium, nickel, tantalum, molybdenum or alloysthereof. The electrical connector 140 comprises a rod or plug 345 havinga length sufficiently long to extend from the voltage supply terminal340, through a hole 350 in the dielectric 1 15 and the support 190, toelectrically engage the electrode 105. Other equivalent structures forthe electrical connector 140 include rectangular leads, contact posts,and laminated conducting structures.

In a preferred structure, shown in FIG. 6, the plug 345 of theelectrical connector 140 is bonded to the electrode 105 by a conductingmaterial in a liquid phase. Preferably, the conducting liquid phasecomprises a metal having a softening temperature of less than about1500° C., and more preferably, less than about 600° C. Suitablematerials include aluminum, copper, iron, molybdenum, titanium, tungstenor alloys thereof. The electrical connector 140 is aligned in the hole350 to provide a gap 355 sufficiently large to allow the conductingliquid phase to infiltrate between and electrically connect the plug 345to the electrode 105. The more ductile conducting material that fillsthe gap 355 also absorbs thermal stresses arising from the verticalexpansion of the electrical connector 140 relative to other surroundingstructures, such as the electrostatic member 100. The volume of gap 355in which the metal is infiltrated is sufficiently large to enable themetal to substantially fill the space between the electrical connector140 and the electrode 105 to provide a good electrical connection.However, it has been discovered that reducing the volume of gap 355 intowhich the metal is infiltrated serves to significantly reduce crackingof the ceramic material surrounding the electrical connector 140 and canalso reduce bowing of the electrostatic member 100. In the embodimentshown in FIG. 6, the gap 355 is defined by a bore 365 in the dielectric115, the bore 365 having a first diameter that is smaller than the outerdiameter of the plug 345 of the electrical connector 140, and a seconddiameter larger than the diameter of the plug 345 to allow it to passthrough. A shoulder 370 defined by the first and second diameters of thebore 365 serves as a stop that prevents the electrical connector 140from contacting the electrode 105, thereby forming a gap 355therebetween that can be infiltrated by molten or softened metal (whichis later solidified) to electrically connect the plug 345 to theelectrode 105. Thus, the electrical connector 140 is not joined directlyto the electrode 105 but instead is electrically coupled via the gap 355filled with a metal which can readily deform and absorb thermalexpansion and other mechanical stresses. This joint provides a morereliable electrical connection between the electrical connector 140 andthe electrode 105.

Alternatively, the electrical connector 140 can be electricallyconnected to the electrode 105 by a brazed connection. Referring toFIGS. 8a and 8 b, a metal insert 375 is placed between the plug 345 andthe shoulder 370 of the bore 365. The electrostatic chuck 55 and theplug 345 are then heated causing the metal insert 375 to soften and fillthe gap 355. Typically, the electrostatic chuck 55 and the plug 345 aremaintained at a temperature of about 6000° C. for at least about 180seconds. Thereafter, they are cooled to solidify the metal in the gap355 to form a brazed connection between the electrical connectors 140and the electrode 105 as shown in FIG. 8b. Optionally, a pressure can beapplied to the plug 345 of the electrical connector 140 while heatingthe electrostatic chuck 55 to cause the softened metal from the metalinsert 375 to infiltrated and fill the gap 355.

Optionally, as shown in FIG. 6, tubes 380 of a ceramic material, such asaluminum oxide, extend through one or more of the dielectric 115, thesupport 190 and the base 175. These tubes 380 serve to electricallyisolate electrical connector 140 and the heater connectors 270 a,b fromthe bond layers 250, 295, the base 175, and the support 190. They alsoalign the conduit 130 and holes 165 a,b through which the lift pins 160pass to prevent the formation of a plasma glow discharge therein duringoperation of the electrostatic chuck 55. The tubes 380 comprise an outerdiameter that allows them to be held in place substantially without theuse of an adhesive. Preferably, the tubes 380 surrounding the electricalconnector 140 and the heater connector 270 a,b comprise an innerdimension and a shape that conforms to the connectors 140, 270 a,b. Morepreferably, the tubes 380 surrounding the conduits comprise an innerdiameter sufficiently small to prevent plasma formation in the conduit130 and in the lift pin holes 165 a,b.

Forming the Base

The version of the base 175 supporting the electrostatic member 100which comprises porous ceramic infiltrated with metal is fabricated byforming a ceramic preform (not shown) and infiltrating a liquid ormolten metal into the ceramic. The ceramic preform is made from aceramic powder having an average particle size that provides the desiredvolume of porosity in the ceramic preform. The average particle size ofthe ceramic powder can be obtained by milling processes, such as ballmilling or attrition milling. The total porosity can be furtherincreased or decreased using agglomerated powder comprising particles ofvarious sizes. Although the desired pore size varies depending on theceramic being infiltrated, it is generally desirable that the ceramicpowder have an average particle size of from about 0.1 to about 50 μm,to yield a volume porosity of from about 20 to about 80 volume %.

The version of the base 175 supporting the electrostatic member 100comprising an embedded heater 235 is formed by placing the resistiveheating element 255 in a mold (not shown), packing the mold with ceramicpowder, and applying a pressure of from about 48 MPa to about 69 MPa tothe mold to form the preform. The pressure applied to the ceramic powdercan be applied using an autoclave, a platen press, or an isostaticpress. Preferably, an isostatic press is used to apply a uniformpressure over the entire surface of the mold to form a ceramic preformhaving high strength. In isostatic pressing, additives such as polyvinylalcohol, plasticizers such as polyethylene glycol, and lubricants suchas aluminum stearate are mixed with the ceramic powder to improve themechanical strength of the preform. Because the preform has sufficientstrength, voids for connectors 140, 270 a,b to the electrode 105 and theresistive heating element 255, the conduit 130 for the heat transfergas, and the holes 165 a,b for the lift pins 160 can be formed usingconventional machining techniques such as drilling, boring, or millingwhile the ceramic preform is in the green state.

The green preform is sintered to obtain a ceramic preform with theoptional resistive heating element 255 embedded therein. In thesintering process, the green preform is heated in the presence of a gasat a high partial pressure in order to control the total porosity andaverage pore size of the sintered body. Preferably, the partial pressureof the gas is from about 1 to about 10 atmospheres. If binders or otherorganic materials are used in the preform forming process, theseadditives are burned out in the sintering step. In the sinteringprocess, the green preform is placed in a furnace and slowly heated to atemperature of from about 300 to about 1200° C. in a flowing gas such asnitrogen to volatilize the organic materials to form a dense ceramic.

The second step of forming the base 175 involves an infiltrationprocess. After a ceramic having the desired total porosity and pore sizeis obtained, a liquid phase of metal or molten metal is infiltrated intothe voids or pores of the ceramic. The infiltration can be accomplishedby any suitable process including, for example, a method in which moltenmetal is brought into contact with a ceramic and infiltrates into theinterconnecting pores of the ceramic by capillary action. In a preferredmethod, infiltration is accomplished in a pressure vessel using apressure infiltration process. In this method, the ceramic is placed inthe pressure vessel with metal around it, and the vessel evacuated andheated to remove air from the pores of the ceramic. Once the pressurevessel is evacuated, the ceramic and surrounding metal are heated to atemperature corresponding to the softening temperature of the metal tobe infiltrated. The molten metal is introduced into the pressure vesselunder pressure to fill substantially all voids, cavities and pores inthe ceramic. For example, in the embodiment wherein the ceramiccomprises silicon carbide having a porosity of about 30%, theinfiltration of molten aluminum is accomplished by maintaining thepressure vessel at a pressure of about 1030 kPa (150 psi), and atemperature of at least 600° C. for about 180 seconds.

Forming the Bond Layers

The base 175 is then bonded to the ceramic dielectric 115 of theelectrostatic member 100 by the infiltration process described above. Ina preferred embodiment, the electrostatic member 100 is placed on top ofthe base 175 in a pressure vessel and molten metal or alloy is broughtinto contact with the assembly. Typically, the process vessel ismaintained at a pressure of from about 690 kPa (100 psi) to about 1380kPa (200 psi), and the molten metal is maintained at temperature of fromabout 600 to about 700° C. for at least about 180 seconds. During theinfiltration process, molten metal reacts with the ceramic dielectric115, forming an intermetallic bond layer 250 between the electrostaticmember 100 and the base 175. After infiltration, the assembledelectrostatic chuck 55 is cooled to solidify the metal to form the bondlayer 250. It has been found that a substantially void-free andcrack-free bond between the electrostatic member 100 and the base 175can be achieved by controlling the rate at which the electrostatic chuckassembly is cooled. Preferably, the electrostatic chuck assembly iscooled at a rate of from about 10 to about 100° C./hr.

In an alternative method, the base 175 is formed and bonded to theelectrostatic member 100 in a single step. In this method, theelectrostatic member 100 with the electrode 105 is placed on thesintered preform of the base 175 in a pressure vessel. Once the pressurevessel has been completely evacuated, a molten metal is introduced intothe vessel under pressure to substantially fill surface voids, cavitiesand pores in the preform to form a base 175 and to also infiltrate intothe interface and bond the base 175 to the electrostatic member 100.

In another embodiment, the support 190 is also bonded to the lowersurface of the base 175 by the infiltration process. As described above,the support 190 can comprise a ceramic or metal structure that is shapedto correspond to the shape of the base 175. The support 190 can beformed by a variety of methods, including for example, casting,isostatic pressing, or machining a block of metal or sintered ceramicmaterial. The cavity 300 is formed in the base 175 by drilling, boring,or milling. For example, in a preferred embodiment shown in FIG. 7c, thesupport 190 is formed from two pieces of cast zirconium. A top member190 a comprises a right cylinder having a cavity 300 with an annularchannel therein, and a lower plate 190 b that covers the cavity 300.Optionally, the lower plate 190 b can also be machined to provide thegas inlet 310 a and the gas outlet 310 b for supplying and exhaustingheat transfer gas from the cavity 300 respectively. After forming thecavity 300, the top and bottom surfaces of the assembled support 190 areground until the surface roughness of the support 190 is less than about1 micron. Surface grinding is needed for the support 190 to uniformlycontact the base 175 and to provide a strong and substantially void freebond layer 295 between the support 190 and the base 175. A smooth bottomsurface is useful to enhance the vacuum seal between the support 190 andthe bottom surface 50 of the chamber 25. After grinding, the support 190is thoroughly cleaned to remove grinding debris. For those embodimentsin which the support 190 comprises a metal, the exposed surfaces of thesupport 190 can be treated or coated with a material to reduce erosionor corrosion by the energized process gases. For example, the exposedsurfaces of the support 190 can be anodized or coated with thermallysprayed alumina.

The following examples illustrate the thermal expansion compatibility ofa variety of combinations of materials that can be used to form theelectrostatic chuck 55, the base 175 and the support 190, or for bondingthe electrostatic member 100 to a base 175 by the bond layer 250. Thetest coupons are scaled down to approximate the dimensions of anelectrostatic chuck 55 and are made from the different materials bondedtogether by the infiltration process of the present invention. Thesilicon carbide and mullite materials were high porosity materialsinfiltrated with a compliant metal, such as aluminum. In theinfiltration process, molten aluminum was infiltrated in a heated andpressurized vessel at a pressure of about 1030 kPa (150 psi) and atemperature of about 600° C.

In Examples 1 to 9, the surface flatness of the bonded test coupons wasmeasured using a profilemeter to determine the degree and direction ofbowing which measures the curvature of a surface from the center to aperipheral edge occurring due to a thermal expansion mismatch of twodifferent materials bonded together. Positive bowing occurs when thecenter of a surface is higher relative to the peripheral edge, andnegative bowing occurs when the peripheral edge is higher. It isdesirable for the receiving surface 120 of the electrostatic chuck 55 tobe flat to prevent breaking of a substrate held to the surface, and toreduce any non-uniformity in the heat transfer rates which occurs whenone portion of the substrate 30 is closer to the electrostatic chuck 55or to the source of the energized process gas. For example, a surface120 having a diameter of about 200 mm should exhibit less than about 254μm (10 mils) of bowing. Excessive bowing can also cause the dielectric115, base 175, support 190, or the bond layers 250, 295 between them tocrack reduce the operating life of the electrostatic chuck 55, orcontaminate the chamber 25.

Referring to Table II, bonded test coupons sized 100 by 180 mm andhaving a thickness of 10 to 12 mm were repeatedly cycled between roomtemperature and a temperature of 300° C. or higher. Subsequent testingand examination demonstrate the ability of the metal-ceramic compositeand the bond of the present invention to securely bond differentmaterials with an acceptable level of bowing and microcracking.

TABLE II EXAMPLE MATERIALS CTE NO. BONDED MISMATCH BONDING QUALITY 1AlSiC to AlN 6.9 to 5.5 Excellent/positive bowing of less than about 10mils. 2 AlSiC to Al₂O₃ 6.9 to 7.1 Excellent/positive bowing of less thanabout 6 mils. 3 AlSiC to 6.9 to 7.9 Excellent/No bowing, Mullite Mullitecracking 4 AlSiC to Ti 6.9 to 9.5 Excellent/positive bowing alloys 5AlSiC to AlSiC 6.9 to 6.9 Excellent/No bowing 6 AlSiC to Metal 6.9 to6.0 Excellent/No bowing (Mo, Ta, W, Kovar and Invar) 7 Al-SiSiC to 5.8to 5.5 Excellent/positive bowing AlN of less than about 2 mils. 8 AlC toAlN 4.8 to 5.5 Excellent/negative bowing of less than about 3 mils. 9AlC to AlC 4.8 to 4.8 Excellent/No bowing

In this manner, the present invention provides a system for holding andsupporting a substrate 30 that is capable of maintaining the substrate30 in a narrow range of high temperatures. The substrate 30 is heated orcooled depending on the heat provided by the plasma and the optionalheater 235. In addition, the electrostatic chuck 55, base 175, andsupport 190 can rapidly heat or cool the substrate 30 without fracturingor microcracking from thermal shock or thermal expansion stresses. Also,the present invention provides a reliable electrical connection betweenthe electrical connector 140 and the electrode 105 of the electrostaticchuck 55.

Although the present invention has been described in considerable detailwith regard to certain preferred versions thereof, other versions arepossible. For example, the electrostatic chuck can be used to hold othersubstrates, such as flat panel displays, circuit boards, and liquidcrystal displays as apparent to those skilled in the art and withoutdeviating from the scope of the invention. Also, the electrostatic chuckof the present invention can be used in other environments, such asphysical vapor deposition and chemical vapor deposition chambers.Therefore, the appended claims should not be limited to the descriptionof the preferred versions contained herein.

What is claimed is:
 1. An electrostatic chuck for holding a substrate,the electrostatic chuck comprising: an electrostatic member comprising adielectric covering an electrode that is chargeable to electrostaticallyhold the substrate; and a base comprising a porous ceramic having a porevolume of from about 20 to about 80 volume %, the base bonded to theelectrostatic member by a bond layer, the bond layer comprising a metalthat is infiltrated into the porous ceramic, and the base furthercomprising a heater capable of raising the temperature of a substrateheld on the electrostatic member by at least about 100° C.
 2. Anelectrostatic chuck according to claim 1 wherein the heater comprises aresistive heating element.
 3. An electrostatic chuck according to claim1 wherein the base comprises a composite of a ceramic and metal, thecomposite having a coefficient of thermal expansion within about ±30% ofa coefficient of thermal expansion of the electrostatic member.
 4. Amethod of fabricating an electrostatic chuck for holding a substrate,the method comprising the steps of: (a) forming an electrostatic membercomprising a dielectric covering an electrode that is chargeable toelectrostatically hold the substrate; (b) forming a base comprising aporous ceramic having a pore volume of from about 20 to about 80 volume%, and a heater capable of raising the temperature of a substrate heldon the electrostatic member by at least about 100° C.; and (c) bondingthe base to the electrostatic member by a bond layer comprising a metalthat is infiltrated into the porous ceramic.
 5. A method according toclaim 4 wherein (c) comprises infiltrating a molten metal into aninterface between the electrostatic member and the base.
 6. A methodaccording to claim 5 comprising cooling the base and electrostaticmember at a cooling rate of from about 10 to about 100° C./hr.
 7. Amethod according to claim 4 wherein (c) comprises placing a brazingmaterial between the electrostatic member and base and heating thebrazing material to a temperature of less than about 600° C. to form thebond layer.
 8. A method according to claim 4 further comprisingembedding a heater comprising a resistive heating element in the base.9. A method according to claim 4 wherein the metal also forms the bondlayer.
 10. A method according to claim 4 comprising forming the porousceramic by sintering one or more of aluminum oxide, aluminum nitride,boron carbide, carbon, cordierite, mullite, silicon carbide, siliconnitride, silicon dioxide and zirconium oxide.
 11. A method according toclaim 4 comprising infiltrating the porous ceramic with a metalcomprising aluminum, copper, iron, molybdenum, titanium, tungsten oralloys thereof.
 12. An electrostatic chuck for holding a substrate, theelectrostatic chuck comprising: an electrostatic member comprising adielectric covering an electrode that is chargeable to electrostaticallyhold the substrate; and a base bonded to the electrostatic member by abond layer, the base comprising a composite of a ceramic and metal, thecomposite comprising a coefficient of thermal expansion within about±30% of a coefficient of thermal expansion of the electrostatic member.13. An electrostatic chuck according to claim 12 wherein the bond layercomprises a metal.
 14. An electrostatic chuck according to claim 12wherein the composite comprises porous ceramic infiltrated with anmetal.
 15. An electrostatic chuck according to claim 14 wherein the bondlayer comprises the same metal as the infiltrated metal.
 16. Anelectrostatic chuck according to claim 12 wherein the base comprises aheater capable of raising the temperature of a substrate held on theelectrostatic member by at least about 100° C.
 17. An electrostaticchuck according to claim 16 wherein the heater comprises a resistiveheating element.
 18. A method of fabricating an electrostatic chuck forholding a substrate, the method comprising the steps of: (a) forming anelectrostatic member comprising a dielectric covering an electrode thatis chargeable to electrostatically hold the substrate; (b) forming abase comprising a composite of a ceramic and metal, the compositecomprising a coefficient of thermal expansion within about ±30% of acoefficient of thermal expansion of the electrostatic member; and (c)bonding the base to the electrostatic member by a bond layer.
 19. Amethod according to claim 18 wherein (c) comprises infiltrating a moltenmetal into an interface between the electrostatic member and the base.20. A method according to claim 19 comprising cooling the base andelectrostatic member at a cooling rate of from about 10 to about 100°C./hr.
 21. A method according to claim 18 wherein (c) comprises placinga brazing material between the electrostatic member and base and heatingthe brazing material to a temperature of less than about 600° C. to formthe bond layer.
 22. A method according to claim 18 wherein the bondlayer comprises a metal.
 23. A method according to claim 18 wherein thebase further comprises a heater capable of raising the temperature of asubstrate held on the electrostatic member by at least about 100° C. 24.A method according to claim 23 comprising embedding a heater comprisinga resistive heating element in the base.
 25. A method according to claim18 comprising forming the base by infiltrating a metal into porousceramic.
 26. A method according to claim 25 wherein the metal also formsthe bond layer.
 27. A method according to claim 25 comprising formingthe porous ceramic by sintering one or more of aluminum oxide, aluminumnitride, boron carbide, carbon, cordierite, mullite, silicon carbide,silicon nitride, silicon dioxide and zirconium oxide.
 28. A methodaccording to claim 25 comprising infiltrating the porous ceramic with ametal comprising aluminum, copper, iron, molybdenum, titanium, tungstenor alloys thereof.